66 research outputs found
Order parameter of MgB_2: a fully gapped superconductor
We have measured the low-temperature specific heat C(T) for polycrystalline
MgB_2 prepared by high pressure synthesis. C(T) below 10 K vanishes
exponentially, which unambiguously indicates a fully opened superconducting
energy gap. However, this gap is found to be too small to account for Tc of
MgB_2. Together with the small specific heat jump DeltaC/gamma_nTc=1.13,
scenarios like anisotropic s-wave or multi-component order parameter are called
for. The magnetic field dependence of gamma(H) is neither linear for a fully
gapped s-wave superconductor nor H^1/2 for nodal order parameter. It seems that
this intriguing behavior of gamma(H) is associated with the intrinsic
electronic properties other than flux pinning.Comment: 7 pages, 5 figures; revised text and figures; references updated,
Phys. Rev. Lett., in pres
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Technical aspects of boron neutron capture therapy at the BNL Medical Research Reactor
The Brookhaven Medical Research Reactor, BMRR, is a 3 MW heterogeneous, tank-type, light water cooled and moderated, graphite reflected reactor, which was designed for biomedical studies. Early BNL work in Boron Neutron Capture Therapy (BNCT) used a beam of thermal neutrons for experimental treatment of brain tumors. Research elsewhere and at BNL indicated that higher energy neutrons would be required to treat deep seated brain tumors. Epithermal neutrons would be thermalized as they penetrated the brain and peak thermal neutron flux densities would occur at the depth of brain tumors. One of the two BMRR thermal port shutters was modified in 1988 to include plates of aluminum and aluminum oxide to provide an epithermal port. Lithium carbonate in polyethylene was added in 1991 around the bismuth port to reduce the neutron flux density coming from outside the port. To enhance the epithermal neutron flux density, the two vertical thimbles A-3 (core edge) and E-3 (in core) were replaced with fuel elements. There are now four fuel elements of 190 grams each and 28 fuel elements of 140 grams each for a total of 4.68 kg of {sup 235}U in the core. The authors have proposed replacing the epithermal shutter with a fission converter plate shutter. It is estimated that the new shutter would increase the epithermal neutron flux density by a factor of seven and the epithermal/fast neutron ratio by a factor of two. The modifications made to the BMRR in the past few years permit BNCT for brain tumors without the need to reflect scalp and bone flaps. Radiation workers are monitored via a TLD badge and a self-reading dosimeter during each experiment. An early concern was raised about whether workers would be subject to a significant dose rate from working with patients who have been irradiated. The gamma ray doses for the representative key personnel involved in the care of the first 12 patients receiving BNCT are listed. These workers did not receive unusually high exposures
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Criticality evaluation for the storage of converter plates in drums
A criticality safety evaluation was performed to support the temporary storage of 20%-enriched uranium converter plates for future use in the Brookhaven Medical Research Reactor (BMRR). A total of twelve such plates each containing approximately one kilogram of the {sup 235}U will be stored in DOT-certified 6M-drums, which have the same dimensions as standard 55-gallon drums except that they are twice as high (178.5cm). Each drum contains a Celotex liner surrounding a central 12.7cm-dia steel pipe. The plates have a nominal size of 0.3cm{times}l0.5cm{times}l25.7cm and fit inside the steel pipe, which extends 130cm in the axial direction. Because the accommodation of twelve plates in one drum is physically possible and more economical, this option for plate storage would be recommended provided that the criticality safety limit is not exceeded. In this paper, the neutron multiplication K{sub eff} in drums is calculated using the Monte Carlo Neutron and Photon Transport code (MCNP). For conservatism, several different configurations which could result.in the most reactive conditions for K{sub eff} have been examined. As part of the effort to optimize the arrangement of plates in drums, a second group of the MCNP calculations is performed using twelve plates evenly contained within two drums placed immediately adjacent to each other model again simulates the most reactive conditions for K{sub eff} estimations
OBSERVATIONS ON FUNGICIDE CONTROL OF WITCHES‘ BROOM, BLACK-POD AND PINK DISEASE OF THEOBROMA CACAO
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